The deposition of thin films, for example, films of silicon dioxide onto a silicon wafer or substrate, may be performed by any one of several well known processes. Such processes include chemical vapor deposition or physical vapor deposition, such as, sputtering. Regardless of the deposition process utilized, the final thickness of the deposited thin film is an important variable which heretofore has been subject to various methods of control.
In a typical thin film deposition process, one or more test wafers or substrates are placed in a vacuum deposition chamber, and the deposition process is executed in accordance with empirically determined process parameters. At the end of the deposition process, the test wafer is removed from the deposition chamber; and the film thickness is measured with measuring equipment. If the measured thin film thickness deposited on the test wafer does not correspond with the desired film thickness, the process parameters are adjusted; and a second test wafer is cycled through the deposition chamber. The above experimental process is repeated until a set of experimentally determined process parameters is found which produce the desired thin film thickness on the test wafers. Thereafter, production wafers are cycled through the deposition chamber utilizing the experimentally determined process parameters.
At predetermined intervals, monitor, or test, wafers are again cycled through the deposition chamber; the deposited thin film thickness measured; and the experimentally determined processing parameters are adjusted so that the deposition chamber continuously produces thin films of the desired thickness. A reiterative thickness calibration process as described above has the disadvantages of being relatively time consuming and inefficient. First, the initial and subsequent calibration processes consume numerous test wafers; and if done extensively, requires several sets of measuring equipment for measuring the test wafer. Second, the production process is halted while the thin film deposition chamber is used for the calibration process during which the initial or subsequent processing parameters are determined and recalibrated during the production run.
Given a set of starting processing parameters that provide the desired thickness of the thin film, there have been several efforts to control the process. For example, as disclosed in the Hurwitt, et al. U.S. Pat. No. 4,957,605, entitled METHOD AND APPARATUS FOR SPUTTER COATING STEPPED WAFERS, with test wafers having surfaces of different geometric characteristics, for example, stepped surfaces, control parameters that determine the operation of the deposition process are varied in accordance with feedback measurements of deposition rate and film thickness. While that apparatus is useful for providing more uniform thin film coating over geometrically diverse surfaces, the disclosed apparatus utilizes the traditional time consuming and inefficient reiterative test sample calibration process. Further, there is no disclosure of controlling the film thickness to a film thickness set point.
The Hurwitt, et al. U.S. Pat. No. 5,126,028 entitled SPUTTER COATING PROCESS CONTROL METHOD AND APPARATUS, discloses a deposition process in which the desired thickness of the coating and the desired deposition rate are entered as input process parameters. Thereafter, other process parameters are calculated, and a thin film deposition process is executed. The thickness of the deposited film on that test wafer is measured; and new process parameters are calculated as a function of the difference between the measured coating thickness and the input parameter representing the desired coating thickness. Those new process parameters are then used to deposit a thin film on the next test wafer to be processed. While this approach provides an improved coating thickness control from one deposition cycle to another, it has the disadvantage of not providing a real time, closed loop set point control of the film thickness within a deposition cycle itself.
Commercially available production control equipment, for example, the model STC-200 thin film deposition controller available from Sycon Instruments, Inc. of East Syracuse, N.Y., provides a capability of terminating the thin film deposition process at a thickness determined by thickness monitoring equipment. The controller utilizes a sensor within the deposition chamber that is located in close proximity to the thin film wafer. Theoretically, as material is deposited on the wafer, a like layer of material is being deposited on the sensor in close proximity to the wafer. The sensor contains a quartz crystal which changes frequency as a function of the thickness of the film being deposited on the crystal. Further, the monitor measures the run time of the deposition cycle, and provides a first output signal which represents the rate at which the film is being deposited on the sensor. A second output signal represents film thickness. While the monitor operates in a satisfactory manner, the monitor measures the thickness of the film on its sensor and not the wafer itself; and therefore, there are limits to its accuracy. Consequently, small variations and inconsistencies within the deposition process, which result in differences between the deposition on the wafer and the monitor sensor, will contribute error to the thickness control.
Another instrument for producing an output signal representing film thickness during the film deposition process is an in-situ spectroscopic ellipsometer of the type available from J. A. Woollam Company, Inc. of Lincoln, Nebr. Spectroscopic ellipsometry has long been known to be a tool for characterizing the physical properties of surfaces, thin films, and multilayer structures. The in-situ ellipsometer is mounted in the deposition chamber so linearly polarized light is directed on and reflected off of the thin film layer being deposited. The reflected light is elliptically polarized, and the optical characteristics of the polarized light are measured. Those measured characteristics are analyzed in a computer within a model of the sample structure to provide output data representing the thickness of the layer being deposited and other optical constants, for example, the refractive index. As with the rate controller previously described, thickness is not directly measured, but is determined as a function of measuring other parameters, for example, with the ellipsometer, the optical characteristics of the surface. While an ellipsometer provides a satisfactory indication of film thickness, the ellipsometer has the disadvantage of not being implemented within a control loop in which the deposition process is controlled to accurately deposit a thin film to the desired thickness in the shortest time.
The above approaches have a further disadvantage in that there is no closed loop control of the ratio of the different reactive gases flowing through the deposition chamber, which ratio is correlated to thin film refractive index.